1. Field of the Invention
The present invention relates to an image forming apparatus such as copier, printer and the like using an electrophotographic system.
2. Description of the Related Art
Conventionally, in a generally available image forming apparatus, it is desired to start printing immediately after the image forming apparatus is powered on or is recovered from a sleep mode. To that end, the temperature of a fixing device needs to rapidly be raised to a predetermined temperature or more, which necessitates increasing the amount of power supplied to the fixing device. On the other hand, the current rating of a receptacle of a commercial AC power source is regulated. Due to this, only a current less than a rated current is allowed to be supplied, which is a problem.
For such problem, there is an image forming apparatus as disclosed in Japanese Patent Application Laid-Open No. 2006-343690. In the image forming apparatus, means to detect current flowing into a fixing device is provided and control is performed to supply more current to the fixing device with less than a rated current.
Also, FIG. 10 illustrates an example of a fixing controller for controlling current flowing into the fixing device of the image forming apparatus. The fixing controller as illustrated in FIG. 10 comprises a main heater 501a, a sub heater 501b, an AC power source 502, a current detection circuit 503, a central processing unit (CPU) 504, a relay 505, a first triac 507, a second triac 506, and a safety circuit 508.
The main heater 501a and the first triac 507 are connected in series. Likewise, the sub heater 501b and the second triac 506 are connected in series. Also, the main heater 501a and the first triac 507 are connected in parallel with the AC power source 502. Likewise, the sub heater 501b and the second triac 506 are connected in parallel with the AC power source 502. The first triac 507 and the second triac 506 are ON/OFF controlled by a heater signal from the CPU 504. The relay 505 is disposed between each heater and the AC power source 502. The relay 505 interrupts power supply to each heater according to a detection result of the safety circuit 508. Each triac is turned ON according to the heater signal from the CPU 504 and power is supplied to each heater by phase control. In this state, the current detection circuit 503 detects the total current flowing into the main heater 501a and the sub heater 502b. As mentioned, the detected current is the total current flowing into the fixing device in a state where the two heaters are simultaneously turned ON.
In a method for detecting the current value as mentioned above, when each triac of two heaters is simultaneously turned on, the current value is detected. Therefore, the detected current value represents the total current value flowing into the two heaters, which prevents accurate detection of the current value for each heater. This arrangement, therefore, leaves a problem that accuracy of phase control performed for every heater cannot be improved. Also, it is desired to individually detect the heater resistance value of each heater. On the other hand, as the number of heaters provided with the fixing device increases, the time required to detect the resistance value also increases. This arrangement, therefore, leaves a problem that the start of printing is delayed.